Relation between ostream and istream. How they can interact?

I am designing a project which has class Encoder having function void write(int p_val) which writes encoded val into std::ostream data-member and class Decoderwhich has void read(int& p_val) from std::istream data-member and overwrites p_val reference. So in order to check whether Encode/Decode process causes data loss or not I want to design another class which will use <boost/iostreams/copy.hpp> to copy std::ostream object's content into std::istream object.

class Encoder{
  public:
     void write(int p_val){
       ...//encodes then writes p_val into m_out
     }
     ...
  private:
     std::ostream& m_out;
     ...
}

class Decoder{
   public:
       void read(int& p_val){
        ...//reads from m_in decodes then overwrites p_val
       }
       ...
   private:
      std::istream& m_in;
      ...
}

Supposing that the design of both classes we can't change, which is the optimal solution to design new class Checker which will check the data-loss in encode/decode process? I found that <boost/iostreams/copy.hpp> gives a way to copy std::ostream into std::istream but I think it isn't the best deal, also having poor skills of multithreading I try to aviod design Checker via multithreading.